emission was assumed, and a universal 1-nsec lifetime was taken to allow us to estimate the fraction of the metastable emitters which decayed within the distance viewed by our X-ray detector. We thus have clear and direct experimental evidence that both two-and three-e,lectron chlorine systems a r e produced which have lifetimes in the nanosecond region and which de-excite at least partially by KCY X-ray emission. We further find that an appreciable fraction (about 60%) of our metastable X-ray emitters belong to systems of more than three electrons. The number of X rays detected from these systems is large because of the charge-state equilibrium at 45 MeV, in spite of the small probability per emerging ion for their production. The appropriate experiment to detect these species directly in a coincidence experiment is in preparation.The identification of the charge states of these X-ray emitters in no way settles the question of their identity. Sellin et al. ' have attributed their metastable Auger-electron emitters to quartet states in three-electron systems whose autoion-X-ray energy with that expected from a two-o r three-electron chlorine system, even though more than half of the associated chlorine ions appear to survive with four o r more electrons, leads us to suspect that the states Seen here may be better attributed to metastable two-electron systems which a r e accompanied by further electrons residing in shells of very high principal quantum number. Such a system would have to owe its metastability against Auger processes at least in part to the poor overlap of the wave function of the outer-shell electron with that of the inner shells, and to slow radiative progression from outer to inner shells by soft photon emission. Experimental observation of the radiation which should accompany such a progression and deduction of the overall excitation states of the outer shells of foil-excited ions would clearly be of importance in helping such an hypothesis to emerge from the speculation stage.ization is inhibited by spin selection rules. They seconds and thus a r e probably not the states we %. L. House, Astrophys. J., Suppl. Ser. E, 21 observe. The consistency of the measured K f f (1969). Solution of the Dirac Equation for Strong External Fields*Berndt ~Ü l l e r , Heinrich Peitz, Johann Rafelski, and Walter Greiner The 1s bound state of superheavy atoms and molecules reaches a binding energy of -2 m c 2 a t 2% 169. It is shown that the K shell is still localized in r space even beyond this critical proton number and that it has a width r (several keV large) which is a positron escape width for ionized K shells. The Suggestion is made that this effect can be observed in the collision of very heavy ions (superheavy molecules) during the collision. The discrete energy eigenvalues for an electron nucleus; the energy eigenvalues a r e then given by bound to a nucleus, which a r e obtained from the the well-known Sommerfeld fine-structure formuDirac equation, lie between moc2 and -moc2, la. In this case th...
In critical o r nearlg-critical heavy-ion collisions, induced as well as spontaiieous eriergyless e -e + pair creation result in the decay of the neutral vacuum. Induced transitions from the negative-energy continuum into a vacant molecular 1s level can occur even in the absence of diving and produce a substantial enhancement and broadening of the previously considered spontaneous positron spectrum. Total c r o s s sections of 5 b have been calculated for C-I! collisions.Intermediate quasimolecules, which will be formed in the collision of very heavy nuclei, show with respect to their electronic structure all properties of superheavy at0ms.l-' In the c a s e that the charge number 2, + Z , of the colliding nuclei is larger than the critical charge number Z u 169-172, the ls, , level enters the negative-energy continuum. If the K shell i s ionized, e'e -creation results. This has to be interpreted a s the decay of the neutral vacuum, which i s unstable in overcritical fields. ' Up to now calculations were done for the spontaneous positron ~r e a t i o n .~ The c r o s s sections will be increased, however, by two effects because of the nonadiabacticity of the heavy-ion collision: First, during the "diving" process e t e -pairs a r e created, in addition to the spontaneous ones, by induced autoionization. Second, before and after the diving a large number of positrons will be created by induced transitions from the negative-energy continuum to the ls", level. The latter transitions will occur even if there i s no level-diving during the collision.To calculate the total transition amplitude, all the transitions, from the negative-energy continuum to the ls", level, along the classical ion path must be added coherently . 7 These transitions niay be approximately grouped into first, pre-and after-diving amplitudes C","and second, the duringdiving amplitude C,. Then we have j -t~i d t ,and r ( t v )represents the decay ls", level due to the interaction with the continuum (to be determined later) and t u denotes the "critical" time a t uThich diving occurs. 1 4 2 and I cp a r e the wave functions for the positron (negative-energy continuum) and the l s U l vacancy, respectively. The time integration can be replaced by an integration over clR/z, along the ion hyperbola, where ri' i s the distance of the two ions and z , the radial velocity. The matrix element .iI,(tl) can be computed by expanding the bound state I ~l ( t ' ) )
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